Preheat your oven to 350°F (175°C) and grease a 9-inch square baking dish.
In a large mixing bowl, combine the sliced apples with lemon juice, brown sugar, ground cinnamon, and nutmeg. Toss until coated and spread evenly in the baking dish.
In a separate bowl, mix together the rolled oats, all-purpose flour, granulated sugar, and a pinch of salt.
Add the cold, cubed butter to the oat and flour mixture and work it into the dry ingredients until it resembles coarse crumbs.
Distribute the crumble mixture evenly over the apples in the baking dish.
Bake for 35-40 minutes, or until the apples are bubbling and the topping is golden brown.
Allow to cool for about 10 minutes before serving.
Notes
Serve warm with vanilla ice cream or whipped cream.